Project Summary/Abstract: The objective of this proposal is to establish the molecular identities of a panel of internalizing mesothelioma cell surface antigens that are bound by a panel of novel internalizing human single chain antibodies (scFvs) that target both epithelioid and sarcomatoid (a particularly recalcitrant form) mesothelioma. The proposed study is built upon our recent work where we have selected a combinatorial human antibody library on live mesothelioma cells and identified a panel of internalizing scFvs that bind to all subtypes of mesothelioma cells in situ with no binding to normal mesothelium, and mediate efficient intracellular delivery of small molecule payloads to both epithelioid and sarcomatoid mesothelioma cells in vitro. We hypothesize that these scFvs define novel mesothelioma antigens that have significantly greater specificity, subtype coverage and therapeutic potential than currently known antigens. Identification of these novel mesothelioma antigens would (1) allow further engineering of the lead antibodies to improve affinity, specificity, pharmacokinetics and biodistribution, (2) allow development of additional antibodies targeting non-overlapping epitopes of the same antigen, which is often required for diagnostic assay development, (3) enhance our understanding of tumor behaviors that involve the cell membrane, which may lead to identification of additional novel targets for further therapeutic development, and (4) provide a sound rationale for clinical trial design such as pre-screening of patients based on target expression. We are uniquely positioned to accomplish this goal as we have developed and adapted effective methods to identify tumor antigens targeted by scFvs. Key to this proposal is our newly developed novel antigen identification strategies based on eukaryotic cell surface display of the human proteome. In addition, we have established functional collaborations with Drs. Burlingame and Chalkley and their National Mass Spectrometry Facility at UCSF to identify tumor antigens that are difficult to identify by expression cloning methods such as post-translationally modified antigens. We propose to use the methods and strategies that we have developed and adapted to systematically identify mesothelioma cell surface antigens targeted by our panel of internalizing human scFvs.